1. Field of the Invention
The present invention relates to a constant speed drive apparatus for an airborne generator which drives an airborne generator with engine torque, and to an aircraft power generating apparatus including the generator. Furthermore, the invention relates to a traction speed change apparatus of continuously-variable speed change ratio.
2. Description of the Related Art
An aircraft has generators mounted thereon for supplying electric power to electrical equipment including instruments, communication devices, lighting, air-conditioners and anti-icing heaters. Each engine of a large passenger airplane, e.g., a twin-engined jetliner, is mounted with a large generator of the order of 100 kVA. The engine has an accessory drive shaft for dividing out turbine torque to drive various devices such as a hydraulic pump. This accessory drive shaft serves as a drive source of the generator.
The engine speed of an aircraft varies widely from idling conditions at the time of landing to acceleration conditions at the time of takeoff, and generally varies continuously over the range of 5,000 rpm to 10,000 rpm. On the other hand, an AC output of 400 Hz.+-.7 Hz is specified for generators, and the drive shaft of the generator is required to rotate constantly at a rate of 12,000 rpm or 24,000 rpm. Thus, in the case where engine output is directly connected to the generator through an accelerating gear or the like, some compensation system is needed since otherwise the rotational frequency of the generator varies to excess.
The following compensation systems have been in wide use heretofore: 1) mechanical type: a hydromechanical CSD (Constant Speed Drive) having a stepless speed change mechanism with a combination of a hydraulic pump, a hydraulic motor and a differential gear, disposed between the accessory drive shaft and a generator to absorb variations in the engine speed and drive the generator at constant speed, and 2) electrical type: a VSCF (Variable Speed Constant Frequency) having a semiconductor device for switching generator output with frequency variations to convert the output into an AC output of fixed frequency.
The former, mechanical type requires a large number of components and a complicated mechanism. This type can achieve only a low transmission efficiency of about 65% due to losses in conversion to hydraulic pressure and the like, and is expensive at that.
The latter, electrical type, though increasingly used at present, becomes heated to a very high degree since it controls a large amount of electric power. Thus, a large cooling mechanism is required, whereby this type is heavier as a whole than the mechanical type. Moreover, since the semiconductor device is less reliable than the mechanical type, the mechanical type remains the preferred option in the case where priority is given to safety and weight reduction.